It is important that developers using memcached understand a little bit about how it works internally. While it can be a waste to overfocus on the bits and bytes, as your experience grows understanding the underlying bits become invaluable.
Understanding memory allocation and evictions, and this particular type of LRU is most of what you need to know.
Memory assigned via the -m commandline argument to memcached is reserved for item data storage. The primary storage is broken up (by default) into 1 megabyte pages. Each page is then assigned into slab classes as necessary, then cut into chunks of a specific size for thatslab class.
Once a page is assigned to a class, it is never moved. If your access patterns end up putting 80% of your pages in class 3, there will be less memory available for class 4. The best way to think about this is that memcached is actually many smaller individaul caches. Each class has its own set of statistical counters, and its own LRU.
Classes, sizes, and chunks are shown best by starting up memcached with -vv:
$ ./memcached -vv
slab class 1: chunk size 80 perslab 13107
slab class 2: chunk size 104 perslab 10082
slab class 3: chunk size 136 perslab 7710
slab class 4: chunk size 176 perslab 5957
slab class 5: chunk size 224 perslab 4681
slab class 6: chunk size 280 perslab 3744
slab class 7: chunk size 352 perslab 2978
slab class 8: chunk size 440 perslab 2383
slab class 9: chunk size 552 perslab 1899
slab class 10: chunk size 696 perslab 1506
In slab class 1, each chunk is 80 bytes, and each page can then contain 13,107 chunks (or items). This continues all the way up to 1 megabyte.
When storing items, they are pushed into the slab class of the nearest fit. If your key + misc data + value is 50 bytes total, it will go into class 1, with an overhead loss of 30 bytes. If your data is 90 bytes total, it will go into class2, with an overhead of 14 bytes.
You can adjust the slab classes with -f and inspect them in various ways, but those're more advanced topics for when you need them. It's best to be aware of the basics because they can bite you.
Memcached uses chunks of memory for other functions as well. There is overhead in the hash table it uses to look up your items through. Each connection uses a few small buffers as well. This shouldn't add up to more than a few % extra memory over your specified -m limit, but keep in mind that it's there.
Memory for an item is not actively reclaimed. If you store an item and it expires, it sits in the LRU cache at its position until it falls to the end and is reused.
However, if you fetch an expired item, memcached will find hte item, notice that it's expired, and free its memory. This gives you the common case of normal cache churn reusing its own memory.
Items can also be evicted to make way for new items that need to be stored, or expired items are discvered and their memory reused.
An item will use space for the full length of its key, the internal datastructure for an item, and the length of the data.
You can discover how large an Item is by compiling memcached on your system, then running the "./sizes" utility which is built. On a 32bit system this may look like 32 bytes for items without CAS (server started with -C), and 40 bytes for items with CAS. 64bit systems will be a bit higher due to needing larger pointers. However you gain a lot more flexibility with the ability to put tons of ram into a 64bit box :)
Slab Stats 56
Thread stats 176
Global stats 108
Item (no cas) 32
Item (cas) 40
Libevent thread 96
libevent thread cumulative 11472
Thread stats cumulative 11376
Items are evicted if they have not expired (an expiration time of 0 or some time in the future), the slab class is completely out of free chunks, and there are no free pages to assign to a slab class.
Memory is also reclaimed when it's time to store a new item. If there are no free chunks, and no free pages in the appropriate slab class, memcached will look at the end of the LRU for an item to "reclaim". It will search the last few items in the tail for one which has already been expired, and is thus free for reuse. If it cannot find an expired item however, it will "evict" one which has not yet expired. This is then noted in several statistical counters.
Memcached uses libevent for scalable sockets, allowing it to easily handle tens of thousands of connections. Each worker thread on memcached runs its own event loop and handles its own clients. They share the cache via some centralized locks, and spread out protocol processing.
This scales very well. Some issues may be seen with extremely high loads (200,00+ operations per second), but if you hit any limits please let us know, as they're usually solvable :)